Hey Cheesefood:
Compression fittings are hard to mess up. Take an adjustable wrench and tighten the packing nut (around the valve stem ). If that doesn't stop the leak you can hold the valve with one wrench and turn the compression nut another half turn.
If that doesn't work, its time to take it off, cut the copper and start over with a new ferrule.
Glenn

Hey Cheesefood:
Compression fittings are hard to mess up. Take an adjustable wrench and tighten the packing nut (around the valve stem ). If that doesn't stop the leak you can hold the valve with one wrench and turn the compression nut another half turn.
If that doesn't work, its time to take it off, cut the copper and start over with a new ferrule.
Glenn